LANZHOU, July 21 -- Police have detained seven people in connection with a gas explosion at a dormitory in Lanzhou University in northwest China's Gansu Province on Monday morning, local work safety watchdog said Tuesday.

The seven will be put under administrative detention for 15 days.

So far, 21 out of the 28 people hospitalized by the blast have been discharged from hospital.

The blast occurred at 7:32 a.m. on Monday at Lanzhou University in Lanzhou, capital of Gansu, leaving 31 people injured. A total of 193 houses near the dorm had their doors and windows damaged in the explosion.

An initial probe suggested that the blast was caused by a gas leak from damaged pipes in an adjacent building. The explosion happened while workers were demolishing the building and accidentally cracked the pipes.

Lanzhou University and the Gansu Provincial Ninth Construction Group Co., Ltd. started the demolition work without any formality or license.

Though the leaking gas was smelt at 2 p.m. on Sunday, no measures were taken in the following 17 hours.